Measurement of the 60Co gamma-gamma Angular Correlation with an Unsegmented Liquid-Scintillator Detector

Abstract We report a measurement of the gamma-gamma angular correlation in the 60Co cascade using a compact coincidence setup based on two unsegmented liquid-scintillator (LS) detector modules read out by photomultiplier tubes (PMTs). The beta-minus decay of 60Co produces two prompt gamma rays of 1173 keV and 1332 keV in the well-known 4+ → 2+ → 0+ cascade, whose angular correlation can be described by an even-order Legendre expansion. Although unsegmented LS detectors do not provide event-by-event interaction-point information as segmented crystal scintillators or HPGe arrays do, the correlation can be recovered statistically from coincidence timing in a fixed detector geometry. We fit the measured angular-correlation distribution with W(θ) = 1 + A2P2(cos θ) + A4P4(cos θ), yielding A2 = 0.1018 ± 0.0146 and A4 = 0.0116 ± 0.0183. These values are consistent with the theoretical coefficients $A_2^{\rm th}=0.102041$ and $A_4^{\rm th}=0.009070$ for a pure E2–E2 cascade. This work indicates that gamma-gamma angular-correlation coefficients can be quantitatively extracted with a compact, unsegmented LS system. It provides an initial benchmark for large homogeneous scintillation geometries in which event-by-event angular tagging is unavailable, and supports the feasibility of future coincidence applications with doped LS or water-based liquid scintillator (WbLS) media.